When a patient is unable to breathe on his or her own due to a critical illness or injury, it becomes necessary for a clinician to place an endotracheal tube (also referred to herein as “ETT”) into that patient's trachea to facilitate the patient's breathing. Similarly, when a patient is unable to breathe independently because s/he is under general anesthesia for surgery, it becomes necessary for a clinician to place an endotracheal tube into that patient's trachea to sustain the patient's breathing.
When preparing to manage a patient's airway, it is important to determine the correct size endotracheal tube for any individual patient for several reasons. As discussed in greater detail below, two parameters are critical—tube diameter and tube length. Tube diameter is measured relative to the narrowest diameter of the upper airway, the cricoid ring. However, the glottic aperture, a triangular shaped opening to the trachea, is defined by the true vocal cords and arytenoid cartilage and is located just proximal to the cricoid ring. The glottic aperture can be measured and the diameter of the cricoid ring can then be calculated. Tube length is measured relative to the distance between the patient's vocal cords and the carina of the trachea (a cartilaginous ridge within the trachea that runs antero-posteriorly between the two primary brochi at the site of the tracheal bifurcation at the lower end of the trachea).
For purposes of reference, two oppositely disposed ends of a tube shall be referred to as the machine end and the patient end, the machine end being the end that remains outside a patient's mouth for connection to a ventilation source (i.e. bag valve mask or mechanical ventilator), and the patient end being the end that is placed into the trachea. Also, for purposes of reference, the trachea can be divided into three theoretical zones. Zone 1 will be referred to as the Upper Trachea—Unsafe Positioning Zone. It is 3.1 cm long and is made up of the 1 cm long cricoid ring immediately below the vocal cords and a 2.1 cm length of the trachea. Research shows that if the machine end of the endotracheal tube cuff (balloon) encroaches on this region, increased risk arises for recurrent laryngeal nerve impingement or pressure directly applied to the vocal cords, either of which may lead to an increased risk for vocal cord injury and paralysis. Encroachment on this region also leads to an increased risk for unplanned extubation. Zone 2 is the Lower Trachea—Unsafe Positioning Zone. It is 2.0 cm long and if the tip of the tube encroaches on this region, an increased risk for endobronchial mal-positioning and associated complications arises. Zone 3 is the Safe Positioning Zone and lies between Zones 1 and 2.
If an endotracheal tube having too large a diameter (relative to the patient's glottic aperture) is placed through the glottic aperture, the force applied to the vocal cords may cause a subluxation or dislocation of the crico-arytenoid joints leading to vocal cord dysfunction. Too small a diameter tube may lead to air leaks and inadequate ventilation of the patient. Similarly, if an endotracheal tube whose length from the tip of the tube at the patient end (T) to the machine end of the balloon or endotracheal tube cuff (BME) (illustrated as T-BME in FIG. 1) is too long relative to the length of the patient's tracheal Safe Positioning Zone, either the tip of the tube will encroach on the Lower-Trachea Unsafe Positioning Zone or the machine end of the inflated balloon will encroach on the Upper Trachea—Unsafe Positioning Zone. All of these conditions place the patient at risk for a number of complications arising from tube mal-positioning including pulmonary atelectasis, hypoxemia, pneumonia, pneumothorax, vocal cord injury, vocal cord paralysis, brain injury and death. To safely place an endotracheal tube in the proper position of the trachea, both the tip of the tube and the balloon must be positioned completely within the patient's Safe Positioning Zone.
Both the outside diameter (OD) of the tube relative to the diameter of the glottic aperture and the (T-BME) length of the endotracheal tube relative to the length of the Safe Positioning Zone should be known when determining the size of endotracheal tube that will be used to intubate a patient to minimize complications of endotracheal intubation and airway maintenance. Historically, however, endotracheal tube sizes and identification nomenclatures have been based solely upon the interior diameter (ID) of the endotracheal tube. Although it is important for the physician to determine the correct endotracheal tube size for every individual patient, most clinicians responsible for the intubation determine endotracheal tube size based upon an educated guess, rather than upon scientific formula, algorithm or accurate measurement of any kind. Some practitioners will choose to place a 7.5 mm endotracheal tube for all females and an 8.0 mm endotracheal tube for all males. Some will choose a 7.0 mm tube for small adults, a 7.5 mm tube for medium size adults and an 8.0 mm tube for large adults. Others may just get a so-called “feel” for the “appropriate” size tube they think a person may need based on their physical characteristics such as height, weight and general size appearance. No generally accepted and widely utilized method, formula, or system exists that maximizes the probability of choosing the optimally-sized endotracheal tube for adults.
In contrast, certain formulas and methods exist that are generally accepted and used by clinicians to calculate the “proper” size tube for insertion into neonates, infants and children. One generally accepted formula based on the age of the child is given as (ETT Size=4+age in years/4), and both weight-based and length-based systems are generally accepted and utilized to choose tube sizes in neonates, infants and children. However, even the weight and length-based systems that are considered the gold standard methods for choosing pediatric size tubes use indirect measures (weight/length) that do not correlate highly to nor predict well the tracheal length and glottic opening diameter and, thus, are not great predictors of optimal tube size.
As noted above, the size of an endotracheal tube is currently defined based on the inside diameter (I.D.) of the tube. Tube sizes range from a size 2.5 mm I.D. to a 10.5 mm I.D in 0.5 mm increments. However, endotracheal tubes with the same inside diameter (I.D.), have varying outside diameters (O.D.) depending upon the manufacturer and tube type. For instance, the Rusch 7.5 mm Standard ETT has an O.D. of 10.0 mm; the Mallinckrodt 7.5 mm Standard ETT has an O.D. of 10.2 mm; the Mallinckrodt 7.5 mm Hi-Lo Evac ETT has an O.D. of 11.2 mm; and the Teleflex 7.5 mm and the ISIS ETT each have an O.D. of 11.3 mm.
The International Organization for Standardization (ISO) requires that both the inside diameter (I.D.) and outside diameter (O.D.) be clearly marked on every endotracheal tube. Despite this reference to the outside diameter, most clinicians do not consider the outside diameter marking on the tube to determine the size of tube that will be utilized for any individual patient.
The ratio of the outside diameter of the endotracheal tube relative to the glottic aperture must be considered in order to minimize the risk for vocal cord injury. This ratio should be less than one. Preferably, the largest diameter endotracheal tube possible (which will minimize the “work of breathing”) should be used while not placing a tube so large that it causes significant pressure on the vocal cords or dislocation of the arytenoid cartilages (leading to vocal cord dysmobility). Ensuring that the diameter of the ETT is smaller than the diameter of the glottic aperture will decrease the risk of vocal cord paralysis from arytenoid cartilage dislocation and other complications as noted hereinabove.
Historically in determining tube size based upon the diameter of the tube, the assumption is made that if the appropriate diameter tube is chosen, the appropriate length of tube automatically follows. However, determination of optimal endotracheal tube size for any individual patient should be based upon considerations of both diameter and length. More specifically, the clinician should consider not only the outside diameter of the ETT relative to the size of the patient's glottic aperture, but also should consider the T-BME length relative to the VC-C length. The VC-C length is defined as the distance from a patient's vocal cords to the patient's tracheal carina. Every patient, based on his or her tracheal length, has a Safe Positioning Zone within the trachea, which defines the region within which both the endotracheal tube tip and balloon must be positioned.
In an attempt to protect patients from vocal cord injury from tubes whose T-BME length is too long, ISO Standard 5361-1999 dictates to manufacturers the maximum allowable distance (DMAX) from the tip of the patient end of an endotracheal tube to the machine end of the inflatable length of the tube's balloon. The ISO Standard DMAX for all size tubes is shown in Table 1. Because the maximum distance rather than the exact distance is defined in the ISO standard, this distance may vary for the same size tube from one manufacturer to another. The ISO Standard simply controls the T-BME length for a given diameter tube. However, even if the clinician chooses a tube having the correct diameter tube, the T-BME length may still be too long, despite ISO standards.
TABLE 1ISO Standard for Max T-Bme Distance (Dmax)I.D. (mm)Dmax (mm)2.0—2.5—3.0—3.5—4.0—4.5—5.0565.5566.0586.5627.0667.5698.0728.5759.078
In order to assist clinicians in placing an endotracheal tube at the correct depth, many manufacturers place a depth localizer band or marker on their endotracheal tubes. The depth localizer bands indicate the position of the tube that should be placed at the level of the vocal cords. Although ISO standards permit depth localizer markers on endotracheal tubes to provide assistance in positioning the tracheal tube within the trachea, no specific standards exist for the placement of these bands on the tube body. Moreover, no standards exist for determining VC-T distances for different size (I.D.) tubes.
As shown in FIG. 2, when the proper size (length) endotracheal tube is placed with the localizer band at the vocal cords, the tip of the tube as well as the entire balloon should be within the Safe Positioning Zone 46. This Safe Positioning Zone preferably places the tip (patient end of the tube) at least 2 cm above the carina to minimize the risk of endobronchial positioning of the tube, should the tube move either due to inadequate stabilization or due to flexion/extension of the patient's neck. It also preferably places the machine end of the inflated balloon at least 3.1 cm below the vocal cords, thus minimizing the risk of impingement of the recurrent laryngeal nerve and vocal cords as well as minimizing the risk of unplanned extubation.
Ensuring that both the tip of the tube and the entire balloon are within the Safe Positioning Zone of the trachea will minimize the risk of complications due to mal-positioning of the tube either at the time of placement of the tube or subsequently should any movement of the tube occur. If any of the T-BME complex is too deep, the patient is at increased risk for endobronchial intubation and any of its inherent complications including hypoventilation, hypoxemia, pneumonia, and pneumothorax. If the T-BME complex is too shallow, the patient is at increased risk for the inflatable balloon impinging on the recurrent laryngeal nerve and/or vocal cords and the inherent complications of vocal cord paralysis. In addition, if the T-BME complex is too shallow, the patient is at increased risk for unplanned extubation and its inherent potentially deadly complications including vocal cord injury/paralysis, aspiration pneumonia, hypoxemia, brain injury and death.
The length of the trachea, from the upper end at the cricoid ring to the lower end at the carina varies in adults from approximately 10 cm to 15 cm with the average adult trachea measuring approximately 12.5 cm. FIG. 3 illustrates the importance of the VC-T and T-BME distances when an endotracheal tube is placed. As shown in FIG. 3, a 7.5 mm ETT is positioned with the depth localizer bands at the vocal cords in (3A) a short trachea (10 cm), (3B) an average trachea (12.5 cm) and (3C) a long trachea (15 cm). Note that both the tip and the entire balloon of the 7.5 mm tube is within the Safe Positioning Zone 46 in both (c) the long trachea (15 cm) and (b) the average trachea (12.5 cm). However, it is outside the Safe Positioning Zone 46 and at risk for endobronchial mal-positioning in (a) the short trachea (10 cm).
As shown in FIG. 4, when a 7.5 mm ETT is positioned in a patient with a short 10 cm trachea based upon the manufacturer depth localizer band properly placed at the vocal cords, the tip of the tube is noted to be too deep and is well within the Lower Trachea—Unsafe Positioning Zone, putting the patient at increased risk for endobronchial mal-positioning. If once the tip is noted to be too deep and the ETT is withdrawn several centimeters so that the tip of the tube is within the Safe Positioning Zone 46, then the machine end of the balloon encroaches on the Upper Trachea—Unsafe Positioning Zone, putting the patient at increased risk for impingement of the vocal cords and laryngeal nerve and increased risk for unplanned extubation. Therefore, a 7.5 mm ETT, manufactured under current diameter driven specifications, cannot be properly placed in any patient with a short trachea (10 cm) without putting the patient at increased risk for complications. Moreover, the diagrams in FIG. 5 illustrate that an individual with a short trachea (10 cm) cannot accommodate a tube larger than a 6.0 mm (FIG. 5D) with both the tube tip and balloon completely within the Safe Positioning Zone 46. Accordingly, the actual length of a patient's trachea should be determined to ensure that a tube with the correct lengths (VC-T and T-BME) is utilized and the length consideration should be separate from the diameter considerations discussed earlier.
In view of the foregoing, it will be apparent to those skilled in the art from this disclosure that a need exists for an improved method and apparatus for determining the optimal endotracheal tube size for safe intubation of a patient to minimize the risk for mal-positioning of the endotracheal tube and the complications associated therewith and that the optimal tube size must be based upon both tube diameter relative to the narrowest portion of the upper airway and length from the tip to the machine end of the balloon relative to the length of the patients trachea and Safe Position Zone. Moreover, a need exists for a method and device that accurately determines the limiting diameter of the patient's upper airway (cricoid ring/glottic aperture) as well as the length of the patient's trachea.